Patrick Blackett conducted cloud chamber experiments, initially based at the Cavendish Laboratory of Cambridge University, where he collaborated with James Chadwick and visiting Italian physicist Giuseppe Occhialini, before moving first to Birkbeck College in London in 1933, and then the University of Manchester in 1937. He was soon joined there by George Rochester.

E. J. Williams conducted cloud chamber experiments at the University of Liverpool under James Chadwick in 1937, before setting up another chamber at University College of Wales in Aberystwyth in 1938.

Louis Leprince-Ringuet conducted cloud chamber experiments based at the École Polytechnique in Paris.

Pierre Auger was based at the University of Paris where he collaborated with Roland Maze.

Bruno Rossi conducted coincidence counter experiments beginning in the early 1930s, based first at the University of Florence (where Occhialini was his first student), then at the University of Padua. In 1938, he fled anti-Semitic persecution in fascist Italy, working first in Manchester in England, then in America at the University of Chicago and at Cornell University before finally joining the atomic bomb development effort at Los Alamos in 1943.

Franco Rasetti, Gilberto Bernardini, and Giuseppe Occhialini were also senior figures in the establishment of coincidence counter studies of cosmic rays in Italian universities; younger experimenters included Oreste Piccioni, Marcello Conversi, and Ettore Pancini. Bernardini was at the Kaiser Wilhelm Institute in Berlin from 1934 to 1937, then moved briefly to the University of Camerino, before settling at dual positions in Rome and Bologna in 1938. In 1937 Occhialini moved from Florence to the University of São Paolo in Brazil. Rasetti worked in Rome before moving in 1938 to Université Laval in Canada.

Paul Kunze conducted cloud chamber experiments based at the University of Rostock in Germany.

Lajos Jánossy experimented using coincidence counters at the University of Berlin before joining the group at Manchester in the late 1930s.

The Appearance of the Mesotron

1935: In January, Japanese physicist Hideki Yukawa publishes a paper positing a short-lived particle intermediate in mass between an electron and a proton to serve as a force carrier that would both hold the nucleus together and account for the beta decay process

1937: At Caltech, Carl Anderson and Neddermeyer publish their identification of the "penetrating" component of cosmic rays as particles of intermediate mass (muons), which they call "mesotrons" (see abstract).

1937: At Harvard, Curry Street and Edward Stevenson report cloud chamber evidence of particles that cannot be classified as protons or electrons, and estimate the mass of the apparently new particle

Resource: Peter Galison, "The Discovery of the Muon and the Failed Revolution against Quantum Electrodynamics," Centaurus 26 (1983): 262-316; or Galison, How Experiments End (Chicago: University of Chicago Press, 1987), chapter three.

1937:Robert Oppenheimer and Robert Serber at Berkeley suggest that the particles detected by Anderson, Neddermeyer, Street, and Stevenson are Yukawa's force carrier; their paper also introduces Yukawa's idea to the western liteature

Resource: Daniela Monaldi, "Life of μ: The Observation of the Spontaneous Decay of Mesotrons and Its Consequences, 1938-1947," Annals of Science 62 (2005): 419-455.

1946: In the latest in a set of coincidence counter experiments conducted in Rome since the middle of the war to establish properties of the meson, Oreste Piccioni, Marcello Conversi, and Ettore Pancini demonstrate decay and absorption rates, which, according to a 1947 theoretical analysis by Enrico Fermi, Edward Teller, and Victor Weisskopf (see citation), are incompatible with a Yukawa particle's properties, as calculated by Tomonaga and Araki

1947: At the physics conference at Shelter Island on Long Island, Robert Marshak proposes the need for a second meson; the hypothesis was soon published in a paper by him and Hans Bethe (see citation). Related ideas had been put forward in Japan during the war by Yasutaka Tanikawa, Shoichi Sakata, and Takesi Inoue.

1949:Robert Leighton, Carl Anderson, and Aaron Seriff analyze muon decay energies and angular distributions to determine that it is spin 1/2, and is accompanied by the production of two neutral particles, probably neutrinos (see abstract).

Postwar Meson Research

1946: At Manchester in England, George Rochester and Clifford Butler detect the forked decay of an unidentified neutral "V" particle (later called K0) in a cloud chamber. In 1947, they detect a charged V particle. However, no one would see these kinds of events again for over two years.

1947: Cecil Powell, Giuseppe Occhialini, Hugh Muirhead, and César Lattes detect a new meson, labeled π, which decays into the known meson, labeled μ, using Ilford nuclear emulsion plates exposed in the Pyrenees and in the Andes. The event was spotted by assistant Marietta Kurz.

Online Resource: Listen to an interview clip of Robert Marshak discussing how the Powell-Occhialini-Muirhead-Lattes observation prompted him to write up his two-meson theory in the midst of his advocacy in postwar atomic politics.

1948: Pions are detected in emulsions in experiments run by Eugene Gardner and Lattes at the new Berkeley 184-inch accelerator (see Accelerators, 1945-1960)

1948: A new heavy meson (τ) is discovered in emulsions sent by Cecil Powell's group to Jungfraujoch in the Alps, which decays into three charged pions; it is published in January 1949

1949:Carl Anderson's cloud chamber group finds 34 V-particle events in observations taken in Pasadena and on White Mountain (see abstract).

1950: Neutral pions (π0) are detected at Berkeley by Bjorklund, Crandall, Moyer, and York in the 184-inch synchro-cyclotron; soon after π0s produced by photons (i.e. "photoproduced pions") are found by Steinberger, Panofsky, and Steller using the electron synchrotron

1950: In October a neutral V-particle that has a proton as a decay product (later termed the Λ0) is discovered by Hopper and Biswas of the University of Melbourne in emulsions flown at 70,000 ft.

1952: Robert Marshak's landmark textbook, Meson Physics, appears.

1953: The Bagnères de Bigorre Conference, dedicated to strange particles and their decay modes. Identified groups are light mesons (L-particles, such as μ and π), heavy mesons (K-mesons, being heavier than the π). The term "hyperon" is coined to describe "mesons" heavier than a neutron (Y-particles, such as the Λ0, formerly the V10).

1955: The new quantum number "strangeness" (S) is proposed independently by Murray Gell-Mann and Kazuhiko Nishijima to help account for the complicated rules now seen as governing particle decay patterns. S is conserved in strong and electromagnetic interactions, but not in weak interactions.

Expected Particle Discoveries

While the proliferation of mesons was an unexpected event that would provide ample questions for the development of elementary particle physics in the ensuing years, the same period also saw the detection of particles that had long been suspected to exist.

1951: At MIT Martin Deutsch detects "positronium" by measuring a delay between nuclear decay and the appearance of an annihilation quantum produced by a positron stopped in a gas (citation). Positronium is a bound state of a positron and an electron, analogous to a hydrogen atom, which can exist briefly prior to their mutual annihilation

1951: On sabbatical from Los Alamos Scientific Laboratory, Frederick Reines decides with Clyde Cowan to attempt to detect the neutrino, which had originally been posited in 1930 by Wolfgang Pauli. Neutrinos were understood to react extremely weakly with matter, making them particularly challenging to detect. Reines suspected a nuclear bomb test might provide a sufficient supply to make detection possible.

1953: Having decided that they could use a nuclear reactor to supply neutrinos, working at the AEC's reactor facility in Hanford, Washington, Reines and Cowan obtain promising evidence of neutrino interaction with protons, producing neutrons and positrons.

1955: A team working at the Berkeley Radiation Laboratory Bevatron accelerator under Owen Chamberlain, Emilio Segrè, Clyde Weigand, and Thomas Ypsilantis detect the antiproton.

1956: Reines, Cowan, and collaborators obtain definitive evidence of a neutrino interaction using a reactor at the new Savannah River facility in South Carolina.